1. Field of the Invention
The present invention relates to a semiconductor optical device applicable to optical communication systems as a light source and more particularly, to a surface-emitting semiconductor optical device such as a Light-Emitting Diode (LED) that includes a transparent semiconductor wafer as a current spreading layer ,and a fabrication method of the device.
2. Description of the Prior Art
A typical semiconductor optical device of this sort is a high-brightness visible LED serving as a light source in the Plastic Optical Fiber (POF)-based optical data link system. This LED may be used as an outdoor display device, an automobile indicator, or a light source in the application fields where an incandescent lamp is usually used.
From the origin of the development of LEDs, lots of research and development have been made for the purpose of improving or enhancing the quantum efficiency of the LED. The internal quantum efficiency of the LED, which is mainly dependent upon the crystal quality, has been able to be achieved up to approximately 100% by using the conventional LED technology.
On the other hand, the external quantum efficiency of the visible LED that has been already achieved is at most approximately 10%. This is mainly due to the inefficient use of the injected current and optical reflection/absorption.
The external quantum efficiency of the LED is dependent on the overall structure of the LED and the shape of an electrode used for spreading the injected current to uniformly flow into the semiconductor p-n junction.
With the conventional surface-emitting LEDs, a thick current spreading layer made of semiconductor is formed on an adjacent cladding layer. The current spreading layer has a function of spreading the injected current through the electrode and of uniformly distributing the injected current into an active layer located on the opposite side to the current spreading layer with respect to the cladding layer. The current spreading layer has a low resistivity for the injected current and a high transmittance for the wavelength of the emitted light.
The injected current will spread within the current spreading layer as the increasing distance from the electrode in a direction perpendicular to the current spreading layer. The spreading range or level of the injected current varies mainly dependent upon the thickness of the current spreading layer. As the thickness of the current spreading layer increases, the injected current is distributed more uniformly, resulting in enhancement of brightness of the LED.
The upper electrode or contact layer needs to be as thin as possible for the purpose of high brightness of the LED.
To decrease the resistivity of the current spreading layer in the conventional surface-emitting LEDs, it is usual that the current spreading layer is heavily doped with a proper dopant. This doping process is typically performed after growing the basic structure of the LED. Any post-growth process such as a high-temperature diffusion process of the dopant may be used to reduce the resistivity.
The formation of the heavily-doped, thick current spreading layer causes several disadvantages stated below.
First, the large thickness of the current spreading layer necessitates the long growth time, which makes the cost of the LED higher.
Second, the heavily doping process of the current spreading layer deteriorates the optical characteristics of the LED. This is caused by the unwanted diffusion of the dopant such as zinc (Zn) into the active layer.
An article, the Japan Journal of Applied Physics, Vol. 31, Part 1, No. 8, August 1992, pp. 2446-2451, disclosed a report about the use of a heavily-doped, thick semiconductor layer as the current spreading layer. The LED reported by this article also has the above first and second problems or disadvantages.
FIG. 1 shows a top view of a typical example of the conventional visible LEDs, and FIG. 2 shows a vertical cross-sectional view thereof along the line II--II in FIG. 1.
As shown in FIG. 1, an n-type GaAs buffer layer 102 is formed on a main surface of an n-type GaAs substrate 1 serving as an optical absorption layer. An n-type (Al.sub.x Ga.sub.1-x).sub.z In.sub.1-z P cladding layer 104 is formed on the n-type GaAs buffer layer 102. A Quantum-Well (QW) active layer 105 is formed on the n-type cladding layer 104. A p-type (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P cladding layer 106 is formed on the active layer 105.
An Al.sub.0.7 Ga.sub.0.3 As current spreading layer 129 is formed on the p-type cladding layer 106. Ap-type GaAs cap layer 130 is formed on the top surface 129a of the current spreading layer 129. The cap layer 130 is contacted with the central area of the top surface 129a. An upper (or, p-side) electrode 131 is formed on the p-type cap layer 130.
A lower (or, n-side) electrode or contact layer 109 is formed on an opposite main surface of the substrate or absorption layer 101 to the buffer layer 102. The lower electrode 109 is entirely contacted with the opposite main surface of the substrate 101.
The exposed top surface 129a of the current spreading layer 129 from the upper electrode 131 serves as an emitting surface from which visible light 132 is emitted.
With the conventional LED shown in FIGS. 1 and 2, the thickness of the current spreading layer 129 also affects the brightness of the emitted light 132, because the optical intensity of the emitted light 132 is directly proportional to the current density on the emitting surface 129a.
The intensity of the emitted light 132 is highest in the vicinity around the upper electrode 131, and it decreases as the lateral distance from the electrode 131 increases. This is because the injected current tends to concentrate at the central region just beneath the electrode 131 within the current spreading layer 129.
The use of the thicker current spreading layer 129 will increase the uniformity of the injected current distribution and the emitted light 132.
FIG. 3 shows the relationship of the light intensity with the position on the emitting surface 129a. When the current spreading layer 129 is comparatively thin, the light intensity varies along the curve 134. This means that the intensity will decrease drastically as the distance from the electrode 131 increases.
On the other hand, when the current spreading layer 129 is comparatively thick, the light intensity varies along the curve 135. This means that the intensity will decrease slightly or gradually as the distance from the electrode 131 increases.
Conventionally, a thick semiconductor substrate transparent to the emitted light 132 has been used with the wafer bonding technology.
An article, the Applied Physics Letters, Vol. 64, No.21, May 1994, pp. 2839-2841, disclosed a report on the enhancement of the external quantum efficiency of the visible LED with the use of a GaP wafer. In this report, a GaAs wafer or substrate serving as an optical absorption layer is etched away and then, the GaP wafer is bonded thereto.
In this LED structure, since the process of etching off the GaAs substrate is required prior to the bonding process, the bonding reproducibility will degrade, resulting in various difficulties in a lot of wafer processes. This leads to fabrication cost increase of the LED.
A large number of articles and Japanese patent publications on visible LEDs (wavelength: 570 nm to 670 nm) based on III-V compound semiconductors were disclosed, in which various LED structures were discussed for enhancement of the external quantum efficiency.
The Japanese Non-Examined Patent Publication No. 2-174272, which was published in 1990, disclosed a high-brightness visible LED. In this publication, the brightness of the LED is enhanced by employing an n-p-n-p structure located under a contact layer, thereby facilitating the spreading of the injected current toward outside the contact layer.
The conventional LED disclosed in the Japanese Non-Examined Patent Publication No. 2-174272 has the following drawback:
Prior to the process of making the contact layer, a mesa structure needs to be formed to reach the underlying active region in the light emitting surface, thereby making a current path. Further, a proper dopant such as zinc is necessary to be diffused in a high-temperature atmosphere. Therefore, the performance of the LED is degraded by the high-temperature heat-treatment for Zn-diffusion, which is due to the deterioration of the active region.
Also, since the light emitting surface contains the heavily-doped dopant, a large part of the emitted light may be absorbed in the diffused layer, which is dependent on the energy of the light.
Further, this conventional LED needs to be fabricated through various processes and therefore, the fabrication cost will be higher.
Additionally, it is difficult to enable the high speed operation because of high parasitic capacitance induced by the wide contact area.
With the above conventional LEDs, the optical output and the coupling efficiency are not high enough for the short-distance data linking systems, especially for the POF-based data linking systems. This is because the conventional LEDs have a low output power and low coupling efficiency with the POF.